CD-ROM (pronounced /ˌsiːˌdiːˈrɒm/ , an acronym of "compact disc read-only memory") is a pre-pressed compact disc that contains data accessible to, but not writable by, a computer for data storage and music playback, the 1985 “Yellow Book” standard developed by Sony and Philips adapted the format to hold any form of binary data.
CD-ROMs are popularly used to distribute computer software, including games and multimedia applications, though any data can be stored (up to the capacity limit of a disc). Some CDs hold both computer data and audio with the latter capable of being played on a CD player, while data (such as software or digital video) is only usable on a computer (such as ISO 9660 format PC CD-ROMs). These are called enhanced CDs.
Although many people use lowercase letters in this acronym, proper presentation is in all capital letters with a hyphen between CD and ROM. It was also suggested by some, especially soon after the technology was first released, that CD-ROM was an acronym for "Compact Disc read-only- media ", or that it was a more "correct" definition. This was not the intention of the original team who developed the CD-ROM, and common acceptance of the "memory" definition is now almost universal. This is probably in no small part due to the widespread use of other "ROM" acronyms such as Flash-ROMs and EEPROMs where "memory" is usually the correct term.
At the time of the technology's introduction it had far more capacity than computer hard drives common at the time, although the reverse is now true though some experimental descendants of it such as Holographic versatile disc may have more space than today's biggest hard drive.
Media
Main article: Compact DiscCD-ROM discs are identical in appearance to audio CDs, and data are stored and retrieved in a very similar manner (only differing from audio CDs in the standards used to store the data). Discs are made from a 1.2 mm thick disc of polycarbonate plastic, with a thin layer of aluminium to make a reflective surface. The most common size of CD-ROM disc is 120 mm in diameter, though the smaller Mini CD standard with an 80 mm diameter, as well as numerous non-standard sizes and shapes (e.g., business card-sized media) are also available. Data is stored on the disc as a series of microscopic indentations. A laser is shone onto the reflective surface of the disc to read the pattern of pits and lands ("pits", with the gaps between them referred to as "lands"). Because the depth of the pits is approximately one-quarter to one-sixth of the wavelength of the laser light used to read the disc, the reflected beam's phase is shifted in relation to the incoming beam, causing destructive interference and reducing the reflected beam's intensity. This pattern of changing intensity of the reflected beam is converted into binary data.
Standard
There are several formats used for data stored on compact discs, known collectively as the Rainbow Books. These include the original Red Book standards for CD audio, White Book and Yellow Book CD-ROM. The ECMA-130 standard, which gives a thorough description of the physics and physical layer of the CD-ROM, inclusive of Cross-interleaved Reed-Solomon coding (CIRC) and Eight-to-Fourteen Modulation, can be downloaded from.
ISO 9660 defines the standard file system of a CD-ROM, although it is due to be replaced by ISO 13490. UDF format is used on user-writeable CD-R and CD-RW discs that are intended to be extended or overwritten. The bootable CD specification, to make a CD emulate a hard disk or floppy, is called El Torito.
CD-ROM drives are rated with a speed factor relative to music CDs (1× or 1-speed which gives a data transfer rate of 150 kiB/s). 12× drives were common beginning in early 1997. Above 12× speed, there are problems with vibration and heat. Constant angular velocity (CAV) drives give speeds up to 20× but due to the nature of CAV the actual throughput increase over 12× is less than 20/12.
CD-ROM format
A CD-ROM sector contains 2,352 bytes, divided into 98 24-byte frames. Unlike a music CD, a CD-ROM cannot rely on error concealment by interpolation, and therefore requires a higher reliability of the retrieved data. In order to achieve improved error correction and detection, a CD-ROM has a third layer of Reed-Solomon error correction. A Mode-1 CD-ROM, which has the full three layers of error correction data, contains a net 2,048 bytes of the available 2,352 per sector. In a Mode-2 CD-ROM, which is mostly used for video files, there are 2,336 user-available bytes per sector. The net byte rate of a Mode-1 CD-ROM, based on comparison to CDDA audio standards, is 44100 Hz × 16 bits/sample × 2 channels × 2,048 / 2,352 = 153.6 kB/s = 150 kiB/s. The playing time is 74 minutes, or 4,440 seconds, so that the net capacity of a Mode-1 CD-ROM is 682 MB or, equivalently, 650 MiB.
A 1× speed CD drive reads 75 consecutive sectors per second.
CD sector contents
- A standard 74 min. CD contains 333,000 blocks or sectors.
- Each sector is 2,352 bytes, and contains 2,048 bytes of PC (mode 1) data, 2,336 bytes of PSX/VCD (mode 2) data, or 2,352 bytes of audio.
- The difference between sector size and data content are the Headers info and the Error Correction Codes, that are big for Data (high precision required), small for VCD (standard for video) and none for audio.
- If extracting the disc in RAW format (standard for creating images) always extract 2,352 bytes per sector, not 2,048/2,336/2,352 bytes depending on data type (basically, extracting the whole sector). This fact has two main consequences:
- Recording data CDs at very high speed (40×) can be done without losing information. However, as audio CDs do not contain a third layer of error correction codes, recording these at high speed may result in more unrecoverable errors or 'clicks' in the audio.
- On a 74 minute CD, one can fit larger images using RAW mode, up to 333,000 × 2,352 = 783,216,000 bytes (~747 MiB). This is the upper limit for RAW images created on a 74 min or ~650 MiB Red Book CD. The 14.8% increase is due to the discarding of error correction data
- The sync pattern for Mode 1 CDs is 0xff00ffffffffffffffff00ff
- Please note that an image size is always a multiple of 2,352 bytes (the size of a block) when extracting in RAW mode.
Manufacture
Main article: CD manufacturingPre-pressed CD-ROMs are mass-produced by a process of stamping where a glass master disc is created and used to make "stampers", which are in turn used to manufacture multiple copies of the final disc with the pits already present. Recordable (CD-R) and rewritable (CD-RW) discs are manufactured by a similar method, but the data are recorded on them by a laser changing the properties of a dye or phase change material in a process that is often referred to as "burning".
Capacity
CD-ROM capacities are normally expressed with binary prefixes, subtracting the space used for error correction data. A standard 120 mm, 700 MB CD-ROM can actually hold about 737 MB (703 MiB) of data with error correction (or 847 MB total). In comparison, a single-layer DVD-ROM can hold 4.7 GB of error-protected data, more than 6 CD-ROMs.
CD-ROM drives
Further information: Optical disc driveCD-ROM discs are read using CD-ROM drives. A CD-ROM drive may be connected to the computer via an IDE (ATA), SCSI, S-ATA, Firewire, or USB interface or a proprietary interface, such as the Panasonic CD interface. Virtually all modern CD-ROM drives can also play audio CDs as well as Video CDs and other data standards when used in conjunction with the right software.
CD-ROM drive can sometimes be a misnomer for newer drives that are capable for reading and burning DVDs, the CD's successor which is now the standard optical disc drive.
Laser and optics
CD-ROM drives employ a near-infrared 780 nm laser diode. The laser beam is directed onto the disc via an opto-electronic tracking module, which then detects whether the beam has been reflected or scattered.
Transfer rates
The rate at which CD-ROM drives can transfer data from the disc is gauged by a speed factor relative to music CDs: 1× or 1-speed which gives a data transfer rate of 150 kiB/s in the most common data format. By increasing the speed at which the disc is spun, data can be transferred at greater rates. For example, a CD-ROM drive that can read at 8× speed spins the disc at up to 4000 rpm (compared to the 500 rpm maximum for 1× speed), giving a transfer rate of 1.2 MiB/s. Above 12× speed, vibration and heat can become a problem. CD-ROM drives above this speed tackle the problem in several ways. Constant angular velocity (CAV) drives spin the disc at a constant rate, leading to faster data transfer when reading from the outer parts of the disc, but slower towards the centre. 20x was thought to be the maximum speed due to mechanical constraints until Samsung Electronics introduced the SCR-3230, a 32x CD-ROM drive wh
Computer Copyright for Family Historians
These notes do not constitute legal advice. My forthcoming Family Tree Magazine article on Copyright for Family Historians will be available as a hand-out, and the computer talk ...
copyright - Computer Definition
Do you have more to add? Share your linguistic knowledge or observation.
Computer Gate - Copyright- California Computer On line Store for PC ...
Computer Gate - Computer On line Store for PC Components and PC Accessories. 2968 Corvin Drive Santa Clara, CA 95051 USA Order Hotline: (888) 437-0895, Other inquiries: (408 ...
Guide to Protection of Computer Software: Copyrights
Protection of computer-related inventions in the United States by Copyright. Ladas & Parry.
Court opens Pandora's computer copyright box - National - theage.com ...
A four-year legal battle over the copyright protection provisions enshrined in the Australia-US free trade agreement has ended in the High Court of Australia with a victory for ...
Xi Computer Corp. Computer - Desktop, Workstation, Server
Custom-build computers using the Xi's online store. CAD computers, and network & web servers. top CAD performance; Xi Computer does it again, for the seventh major reveiw in a row ...
COMPUTER SCIENCE CONCEPTS IN COPYRIGHT CASES: THE PATH TO A COHERENT ...
,240, Harvard Journalof Law & Technology [Vol. 10 I. INTRODUCTION The law surrounding computer software copyright' is on a collision course with Computer science.
Copyright Computer Concepts
Computer Concepts copyright notice. These pages are © Copyright Computer Concepts Ltd 1996. Unless otherwise stated on the page, you are free to copy them, but please include the ...
FAQs from 'Computer Software & Copyright ...
Powered by Plone, the Open Source Content Management System This site conforms to the following standards:
PennTags Development /tag/copyright+computers
feed available here